The present invention relates to a microwave landing system which is protected against jamming.
Before describing the subject matter of the invention, it would appear advisable to describe a microwave landing system (MLS), which is increasingly being used in place of conventional instrument landing systems. Although the latter are widely used throughout the world, they are increasing being found unable to meet the needs of modern aviation, despite the improvements made to them.
A microwave landing system supplies the aircraft with all the information necessary for determining its position relative to the runway in the form of a bearing angle and an elevation angle, together with its distance, the latter being given by an associated distance measurement equipment (DME). The MLS is said to be anametric, because the measurement is performed on-board the aircraft on the basis of information transmitted by the ground MLS station, whose transmission is intended for all aircraft within its coverage. Each aircraft determines its own position on the basis of information transmitted by the station and without entering into bilateral communication with it.
A group MLS installation generally comprises two ground stations, one transmitting a bearing message and the other transmitting an elevation message, a third DME station supplying the distance message. However, throughout the remainder of this text, no mention will be made of the latter station. In addition, no reference will be made to the equipment on board the aircraft, which utilizes the information transmitted by the ground MLS stations. On the basis of this information, the aircraft determines its angular position (bearing and elevation) and its distance from the axis and from a given reference point of the runway.
In principle, a ground elevation and/or bearing angle measurement is performed on the basis of an antenna producing a narrow fan-shaped beam, which scans the angular section of the MLS coverage and the angular position of the aircraft is determined by the outward and return scan of the beam, by measuring the time interval between two pulses received by the aircraft, one for each passage of the beam. The MLS ensures a certain number of functions, but hereinafter significance will only be attached to the bearing and elevation functions. The latter functions, together with the other functions, are transmitted on a time sharing basis. Transmission takes place on a single carrier frequency allocated to the angular station. The angular measurement part, scanning of the beam, is carried out without modulation of the carrier and data transmission takes place in DPSK modulation, modulation by differential phase.
In a MLS system, the transmission relative to a function always starts by a preamble transmitted by a sector coverage antenna, covering the complete MLS volume. This preamble ensures the synchronization of the on-board measuring sequence and gives the identity of the following angular function, i.e. elevation or bearing The transmission of the preamble is thus followed by the transmission of the outward and return scans of the scanning beam produced, in the manner described hereinbefore, by an electronic scanning antenna, the measurement on board the aircraft of the time which has elapsed between two successive passages of the beam striking the aircraft permitting the angular measurement.
A more detailed description of the aforementioned MLS is given in the two following articles:
le MLS, un exemple d'utilisation du microprocesseur de B. Letoquart and J.M. Skrzypczak which was published in the Review "Navigation", April 1981; PA0 the MLS in France, published in the Review "Microwave Journal", May 1981, pp. 113 to 120.
However, in connection with the operating phase of the MLS relating to the transmission of the preamble and then of the angular function, that the sector antenna transmitting the preamble of the function and covering the proportional scan sector has a gain lower that that of the scanning beam antenna. In the case of system jamming, this gain difference between the antennas makes the preamble more vulnerable than the angular measurement signals.